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Model A1224 / Mid 2007 and Early 2008 / 2, 2.4, or 2.66 GHz Core 2 Duo processor

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Is my backlight going out?

The top of my screen is noticeably dimmer now than in the past. "Easy," I thought, "it's the top backlight going out!" But now I'm not so sure. See, whenever the iMac wakes from sleep, the screen is nice and bright for about ¼ second before going back to its now-normal dim state.

Can anyone tell me what's going on? Is my backlight in fact going out, or is something else going wrong? I've checked the brightness control in Displays, and calibrated the screen, with no improvement.

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Bump. Anybody? Bueller?

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Sorry I missed this the first time :-(

Sadly your system uses CCFL's which over time do fade in brightness as they get close to burning out. Just like your office ceiling CFL's do.

While you can still get replacement CCFL's they are a bear to replace and if you're not careful easy to break as well. Here's a good guide to follow: Removing CCFL from iMac 20in LCD CCFL assembly. If you've never attempted anything this complex you might want to opt out and just replace the complete LCD unit. While more expensive it is a lot less work and risk. Here's the IFIXIT guide: iMac Intel 20" EMC 2133 and 2210 Display Panel Replacement.

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iMac Intel 20" EMC 2133 and 2210 Display Panel Replacement

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5 - 20 minutes

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Removing CCFL from iMac 20in LCD CCFL assembly

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1 hour

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But does that explain why the screen will go back to its old brightness for a second after it wakes up?

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Yes, just like a ceiling CFL's they react the same way.

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CFLs slowly brighten as they warm up, not immediately dim right after they're turned on.

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You're over thinking this ;-} When the bulbs are just about worn out they get brighter.

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And then immediately dim? I've never seen that.

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The backlight might be fine. You may need to replace the inverter. I would suggest trying a new inverter before sourcing and replacing the LCD itself. Here is a guide on how to swap it out: iMac Intel 20" EMC 2133 and 2210 Inverter Replacement

iFixit does not have any in stock at the moment, but you might be able to find one on PowerBookMedic.com, Ebay, or Amazon.

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If it was an inverter issue both CCFL's would react in the same way.

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